1. Field of the Invention
The invention relates to a denture cleansing composition and a process for making such a composition.
2. Description of the Related Art
Denture cleansing generally is carried out either by brushing dentures with a paste or by soaking dentures overnight in an aqueous cleansing solution. Aqueous denture cleanser solutions are known and generally comprise tablets, granules, or powders that are dissolved in water to form a cleansing bath or cleansing system in water.
One type of denture cleansing tablet (herein "tablet") uses an effervescent system including sodium bicarbonate, citric acid and/or alkaline proteolytic cleaning enzymes, bleaching or oxidizing agents, such as alkali metal and alkaline earth metal, perborates, e.g. anhydrous sodium perborate and sodium perborate monohydrate (herein "perborates"), and monopersulfates. This type of tablet requires a small amount of water in preparation to assure cohesion of the tablet. The tablet may or may not require curing.
A second type of tablet uses cleaning enzymes and anhydrous sodium perborate in combination with sodium perborate monohydrate. There are no persulfates in this type of tablet. This second system does not require added water to assure cohesion of the tablet. Production of the tablet does not require heat curing.
The cleansing systems produced by both of these tablets when dissolved in water have drawbacks. The pH of the aqueous solution in the first system is too low (i.e. too acidic) for fully effective cleaning enzyme activity. In addition, the reaction of the monopersulfate in the tablet with chlorides in the water produces a hypochlorite which inactivates the cleaning enzymes in the tablet, further depressing their efficacy. The hypochlorite also reacts with expensive fragrances in the tablet, depressing their efficacy.
The use in tablets of perborate monohydrates that do not inactivate proteolytic cleaning enzymes either directly or indirectly is known. However, the use of potassium monopersulfate in the form of "OXONE", in the presence perborate monohydrate, in a weight ratio of approximately 3:1 has been shown to result in the formation of a sufficient amount of hypochlorite in water to deactivate alkaline proteolytic cleaning enzymes. U.S. Pat. No. 5,118,623 to George Boguslawski and John W. Shultz of Solvay Enzyme Inc., issued Jun. 2, 1992, discloses that many cleaning enzymes are inactivated in the presence of chlorine and other halogens. A paper by Waku et al, CA 78 (17):107533 p discloses inactivity of enzymes in the presence of as little as 0.2 parts per million free chlorine.
The pH of the aqueous solution in the second system is too high (i.e. too basic). This high pH has a tendency to destroy the fragrance of the cleaning solution. In addition, the pH of the second type of tablet may be too high for optimal activity of the cleaning enzyme.
There have been efforts, with limited success, to develop a tablet in which deactivation of cleaning enzymes and fragrances do not occur in the tablets, while good denture cleaning efficacy is still provided.
U.S. Pat. No. 4,409,118 to Anthony Eoga, issued Oct. 11, 1983, discloses an effervescent cleansing composition in tablet form comprising: (1) a phosphate salt; (2) a silicate salt; and (3) at least one perborate salt. At least part of the perborate salt is in a compacted granulated mixture with a polymeric fluorocarbon.
U.S. Pat. No. 4,857,224, to Anthony Eoga, issued Aug. 15, 1989, discloses an effervescent cleansing composition in tablet form comprising: (1) a pregranulated and compressed mixture of an anhydrous perborate, a perborate monohydrate and a polymeric fluorocarbon compound, and (2) a monopersulfate compound. This composition is useful for forming a tablet from monopersulfates and anhydrous perborates.